1. Field of the Invention
The present invention relates to a magnetic levitating apparatus for levitating a levitated object (a transportation unit) by utilizing a magnetic attraction force, and more particularly to a magnetic levitating apparatus having an improved apparatus maintainability.
2. Description of the Related Art
Recently, as part of office automation, a transportation apparatus is widely used to move semifinished products and documents between a plurality of points in a building. The transportation apparatus used for such a purpose is required to have a function for quickly and quietly moving articles. Besides, a transportation apparatus used in a very clean space such as a clean room is required to produce no dust in operation.
To meet such requirements, this type of transporting apparatus adopts a system in which a transporting vehicle runs along a guide rail in a non-contact state. In particular, a system in which a transporting vehicle is supported by magnetic attraction force in a non-contact state is excellent in tracking a guide rail and in preventing noise and dust.
In the meantime, in the system wherein the transporting vehicle is supported by magnetic attraction, if all magnetic force needed to support the transporting vehicle is to be produced by electromagnets, the electromagnets must be constantly energized and accordingly the power consumption increases.
Under the circumstances, the inventors proposed a levitating transporting apparatus of a so-called zero-feedback control system (hereinafter referred to as "zero power control") (Jap. Pat. Appln. KOKAI Publication No. 61-102105). In this apparatus, a magnetic support unit is constituted by an electromagnet and a permanent magnet. Most of the magnetic force needed to non-contact supporting is produced by the permanent magnet, thereby reducing power consumption.
In order to stably run the levitating transporting apparatus of the zero power control system, a single magnetic support unit is sufficient to support the transporting vehicle. Normally, two or more magnetic support units (e.g. four units at the four corners of the transporting vehicle) must be provided. When the four magnetic support units are provided, it is desirable that the weight of the entire levitating unit be constantly applied to the magnetic support units in unit of 1/4. In fact, however, there is an imbalance in distribution of the weight. The imbalance results in the following problem.
The four magnetic support units are all fixed to the transporting vehicle. If gap lengths are determined to produce attraction force equal to the weight, which can be supported by the magnetic force of the permanent magnets included in three of the four magnetic support units, a gap length of the remaining magnetic support unit is geometrically determined by the positions of the three magnetic support units. As a result, the actual gap length of the remaining magnetic support unit does not necessarily coincide with a theoretical gap length for producing the attraction force for supporting the weight shared by this magnetic support unit.
For example, a gap length theoretically designed to support a load of 2 kg may be forcibly changed to a gap length for supporting a load of 3 kg by geometrical conditions. Consequently, in order to eliminate a difference in gap length, the electromagnets of the magnetic support units are energized excessively, resulting in an increase in power supplied to the entire electromagnets. Thus, a large-capacity power supply must be provided, and the size of the entire apparatus increases.
To solve this problem, the inventors proposed a magnetic levitation transporting apparatus, described hereunder, having at least four magnetic support units (Jap. Pat. Appln. KOKAI Publication No. 1-45734). Specifically, pairs of magnetic support units are prepared. Each pair of magnetic support units are supported by a separate division plate. The division plates are coupled by a coupling mechanism so as to be rotatable in a plane vertical to the lower surface of the guide rail. Further, the transporting vehicle is fixed to one of the division plates or divided to correspond to the division plate. With this structure, each magnetic support unit can have freedom of movement in the direction of the gap length. Thereby, each gap length can be automatically adjusted to such a value as to produce attraction force needed to support the weight shared theoretically by each magnetic support unit.
However, there remains a problem with this gap-variable mechanism. When a load, regarded as a solid body, is placed on the transporting vehicle of the levitating transporting apparatus having the gap-variable mechanism, the weight of the load is distributed to at most three magnetic support units. The reason is as follows.
When zero power control is executed, the gap length between each magnetic support unit and a guide rail is set at such a value that the sum of the weight of each magnetic support unit and the load applied to each magnetic support unit is equal to the attraction force of each permanent magnet. The gap length between each magnetic support unit and the guide rail decreases if the weight to be supported increases, and it increases if the weight to be supported decreases, as shown in FIG. 8.
Suppose that a load regarded as solid body is placed on the transporting vehicle having the gap-variable mechanism and four magnetic support units. At this time, if the center of gravity of the load itself is eccentric or an external force is applied to the vehicle, the weight of the load applied to the four magnetic support units increases or decreases.
In the magnetic support unit having an increased load weight, the length of gap between this unit and the guide rail decreases. On the other hand, in the magnetic support unit having a decreased load weight, the length of gap between this unit and the guide rail increases. Thus, the support point at which the load weight is applied to the former magnetic support unit shifts upwards, and the support point at which the load weight is applied to the latter magnetic support unit shifts downwards. The entire load weight is unchanged, and the magnetic attraction force between the magnetic support unit and the guide rail is inversely proportional to the square of the gap length. Thus, the weight supported at the downwardly shifted supported point is newly added to the upwardly shifted support point, and the load weight at the downwardly shifted support point is reduced by the amount which is newly added to the upwardly shifted support point. Accordingly, the gap length of the magnetic support unit having the increased load weight decreases further, and the support point at which the load weight has been applied to the magnetic support unit moves further upwards. Moreover, the gap length of the magnetic support unit having the decreased load weight increases further, and the support point at which the load weight has been applied to the magnetic support unit moves further downwards.
This process is repeated, and as a result the load weight is supported by at most three magnetic support units which are at least necessary for supporting the solid body. In this way, even if the load is placed on the transportation vehicle having the gap-variable mechanism and four or more magnetic support units, the load weight is distributed on at most only three magnetic support units if the solidity of the load is high. Thus, in order to fully support the load weight by any three of the magnetic support units, it is necessary to increase the size of each magnetic support unit or to divide one load to distribute the weight thereof. However, since there is a load which cannot be divided, the size of the magnetic support unit must be increased in order to deal with such a load. As a result, the weight of the transporting vehicle is increased, and accordingly the size of the guide rail and track increases. Consequently, the size of the entire apparatus increases.
To solve the above problem, the inventors proposed an apparatus having an automatic load weight distributing mechanism (Jap. Pat. Appln. KOKAI No. 61-170206). Specifically, an elastic body is interposed in the gap-variable mechanism. This elastic body is defined as follows. It is supposed that an inverse number of variation amount of a gap length per unit weight due to a spring force of an elastic body of a magnetic support unit when a load is applied to the magnetic support unit with the load support point fixed is equal or less than an absolute value of a value obtained by differentiating the attraction force in the direction of gap length of the magnetic support unit by the gap length in the state in which an excitation current of the electromagnet is zero and no load is placed on the transportation vehicle. Thereby, automatic distribution of the load weight to each magnetic support unit can be effected.
However, in the apparatus having the above load weight automatic distributing mechanism, there is the following problem. Specifically, since the inverse number of variation amount of a gap length per unit weight due to a spring force of an elastic body of a magnetic support unit when a load is applied to the magnetic support unit with the load support point of the transporting vehicle fixed is equal or less than an absolute value of a value obtained by differentiating the attraction force in the direction of gap length of the magnetic support unit by the gap length in the state in which an excitation current of the electromagnet is zero and no load is placed on the transportation vehicle, the division plates of the transporting vehicle of division structure may be rotated by the attraction force of the magnetic support unit by means of the aforementioned coupling mechanism when the transporting vehicle is removed from the guide rail. In addition, it is possible that the magnetic support unit may be damaged by collision with the guide rail. Besides, in the case of the transporting vehicle wherein the magnetic support unit is attached to the vehicle via an elastic member, the transporting vehicle may be damaged even if the vehicle is lowered since the magnetic support unit exerts attraction force to the guide rail and the elastic member is deformed excessively. Thus, when the transporting vehicle is disengaged from the guide rail for the purpose of maintenance or the like, it is necessary to fix the division plate and all magnetic support units to the vehicle or to lower all the magnetic support units to disengage the transporting vehicle from the guide rail. Consequently, the maintainability of the apparatus is degraded.
As has been described above, in the prior art, the inverse number of variation amount of a gap length per unit weight due to a spring force of an elastic body of a magnetic support unit when a load is applied to the magnetic support unit with the load support point of the transporting vehicle fixed is equal or less than an absolute value of a value obtained by differentiating the attraction force in the direction of gap length of the magnetic support unit by the gap length in the state in which an excitation current of the electromagnet is zero and no load is placed on the transportation vehicle. That is, the mechanical spring force acting on the magnetic support unit is weaker than the magnetic spring force acting on the magnetic support unit when no load is placed on the vehicle. Thus, when the transporting vehicle is disengaged from the guide rail, it is necessary to fix the division plate and all magnetic support units to the vehicle or to lower all the magnetic support units, in order to prevent the apparatus from being damaged. Therefore, it is difficult to perform maintenance of the entire apparatus by disengaging the transporting vehicle from the guide rail. Furthermore, when the transporting vehicle is not disengaged from the guide rail, orbit structural elements such as guide rail becomes an obstacle and maintenance of the entire transporting vehicle cannot be performed.